1. Field of the Invention
The present invention relates to an ammonium salt of an organic acid, and more particularly to a resist composition containing the ammonium salt of organic acid.
2. Description of the Prior Art
A chemically amplified resist is a resist whose degree of dissolution in a developer changes due to the acid generated by exposure. This type of photoresist solution includes a protected resin, a photoacid generator (PAG), and a solvent. The so-called protected resin is a resin that is protected by an acid-labile protective group. The protective group decomposes in the presence of an acid, thus making the resin soluble in an alkali developer.
When the substrate coated with a chemically amplified resist is exposed to light, the photoacid generator will decompose and generate a strong acid, which will catalyze the resin to induce a catalytic hydrolyzation. The protective group (such as an ester group or acetal group) in the resin thus decomposes, making the resin soluble in an alkali developer. In this way, the degree of dissolution of the resist before and after exposure is different, and a pattern can thus be formed.
When the resist is operated in an environment containing a base material, such as hexamethyldisilazane (HMDS) or N-methyl-2-pyrrolidone (NMP), this base material will erode into the surface of the resist. Thus, the acid generated in the exposed area of the resist will be neutralized, which will decrease the concentration of the acid, and the protective groups can not be completely hydrolyzed. The pattern after photolithography thus has adverse phenomenon such as T-top or skin. In addition, the acid generated in the exposed area will diffuse into the unexposed area, thus changing the critical dimension (CD).
To prevent the above problems, either the base concentration in the environment should be strictly controlled, or a base additive is introduced to the resist. In the latter case, the resist as a whole is in basic in nature, which can inhibit the resist from absorbing other base material present in the environment. When the exposure energy is as high as E0 (dose-to-clear energy), a large amount of photoacid will be generated after exposure. A small portion of the photoacid is neutralized by the base additive in the exposed area, while most of the photoacid will catalyze the resin to break the protective groups. Although a small amount of photoacid molecule still goes into the unexposed area through diffusion, the base additive can neutralize the photoacid in the unexposed area. Therefore, the line width change caused by acid diffusion can be prevented.
Since the base additive is introduced mainly for reacting with the photoacid, it is generally called xe2x80x9ckiller basexe2x80x9d. Some researchers have used a primary or secondary amine as a killer base, for example, in U.S. Pat. No. 5,942,367. Such a kind of killer base can effectively increase environmental stability of the resist, and the pattern obtained is good. However, the primary or secondary amine will react with some components in the resist composition, such as those containing anhydride or phenol groups. Thus, the storage stability of the resist is decreased. Some researchers have used a tertiary ammonium salt as a killer base, which is disclosed in, for example, U.S. Pat. Nos. 5,633,112 and 5,411,836. However, it is easily seen that the pattern has a slope problem. Also, the rounding problem occurs due to using only one salt.
Therefore, an object of the present invention is to solve the above-mentioned problems and to provide an ammonium salt of an organic acid. When the salt is used as a base additive for a chemically amplified resist, the environmental stability of the resist can be enhanced, and the T-top phenomenon can be effectively prevented. In addition, the line width change caused by acid diffusion can be prevented, and the E0 value of the resist can be decreased.
To achieve the above objects, the ammonium salt of an organic acid of the present invention has the following formula: 
wherein
R is selected from the group consisting of unsubstituted or substituted cyclic alkyl, cyclic alkenyl, cyclic ester group, and cyclic ketone group having from 3 to 20 carbon atoms,
R1, R2, and R3 are selected from the group consisting of H, unsubstituted or substituted linear or branched C1-C20 alkyl, C3-C10 cyclic alkyl, and unsubstituted or substituted linear or branched C1-C20 alkyl containing an N, O, or S atom,
when R1, R2, and R3 are unsubstituted or substituted linear or branched C1-C20 alkyl, any two of R1, R2, R3 can link together to form a ring, and when R1, R2, and R3 are unsubstituted or substituted linear or branched C1-C20 alkyl containing an N, O, or S atom, any two of R1, R2, and R3 can link together to form a ring containing N, O, or S,
n is an integer of from 1 to 4, and
R1, R2, and R3 are not hydrogen at the same time.